Protecting the piece and preparing the king's return to f7, but...
20. b2c3 A critical moment. 20... b4c3 This move loses. 21. d3f3 A mistake in reply. 21... f7g6 The move goes to the wrong side. 22. d4c6 A deadly blow, certainly missed by Warmerdam. White prepares to bring the rook to g4.